A variety of high-molecular-weight polysaccharides produced by plants (cellulose, pectin and starch), seaweeds (alginate and carrageenan), and bacteria (alginate, gellan, and xanthan) find applications as viscosifying, stabilizing, emulsifying, gelling or water-binding agents in food and non-food industries. All of these polysaccharides are additives, however, and therefore they are considered less desirable in the food industry.
Levans, also known as fructans, β-(2→6)-D-fructoses or β-(2→6)-D-fructans or β-(2→6) -D-fructofuranans or β-(2→6)-D-Fruf or phleins in plants, are natural homopolysaccharides of D-fructofuranose covering a wide range of molecular sizes (˜2 kDa to ˜60 MDa). Low-molecular-weight levans are produced in plants as carbohydrate reserves (Vandamme, E. J., and Derycke, D. G. (1983) Adv. Appl. Microbiol., 29: 139-176). The molecular structure is in this case a short linear chain of β-(2→6)-fructose. Several types of microorganisms, ranging from plant-associated bacteria, to soil bacteria, dental plaque bacteria, yeasts and fungii, also produce levans (Han, Y. W., (1990) Adv. Appl. Microbiol., 35, 171-194): Acetobacter, Actinomyces, Aerobacter, Aspergillus, Bacillus, Corynebacterium, Erwinia, Leuconostoc, Pseudomonas, Streptococcus and others. In microorganisms, levans have higher molecular weights and generally extensive branching through β-(2→1) linkages are present in addition to β-(2→6) linkages seen in plants levans.
Lactic acid bacteria which are food-grade organisms that possess GRAS (generally recognized as safe) status in the USA and are known to produce abundant variety of exopolysaccharide molecules (Cerning, J. (1990) FEMS Microbiol. Rev. 87:113-130; Dunican, L. K. and Seeley, H. W. (1965) J. Gen. Microbiol. 40: 297-308), which contribute to the texture of fermented milk. Exopolysaccharides from these bacteria may allow development of a new generation of food-grade polysaccharides.
Indeed, synthesis of heteropolysaccharides by lactic acid bacteria, including Lactobacilli, is currently being studied intensively (Cerning, J. (1990) FEMS Microbiol. Rev. 87: 113-130 ; Grobben, G. J., et al. (1995) J. Appl. Bacteriol. 79: 103107 ; Stingele, F. et al., (1996), J. Bacteriol. 178: 1680-1690 and van den Berg, D. J. C., et al., (1995) Appl. Environ. Microbiol. 61: 2840-2844).
Also, synthesis of homopolysaccharides (e.g. dextran and levan) has been studied mainly in Leuconostoc mesenteroides (NO 6055) and Streptococci (Cerning, J. (1990) FEMS Microbiol. Rev. 87: 113-130). Limited information is available about homopolysaccharide biosynthesis in Lactobacilli (Dunican, L. K. and Seeley, H. W. (1965) J. Gen. Microbiol. 40: 297-308; Pidoux, M. et al., (1990) J. Appl. Bacteriol. 69: 311-320).
More recently, G. van Geel-Schutten et al. have identified a strain of Lactobacillus reuteri LB 121 which synthesizes water-soluble exopolysaccharide material with both glucose and fructose and further characterized these glucan and fructan exopolysaccharides (van Geel-Schutten, G. H., et al., (1999), Appl. Environ. Microbiol. 65: 3008-3014).
Thus, it would be of interest to provide other strains of Lactobacilli, and especially outside of the Lactobacillus reuteri species, that are able to produce fructans, and especially levan, and that would present a great interest for food industries.